In orthopedic total joint replacement surgery it is necessary for the implant to be securely fixed to the host bone for a successful clinical outcome. Normally this is accomplished in one of two ways; either the implant is fixed to the bone using a surgical cement, or it is forcefully pressed into the bone to achieve an interference fit. When the implant is to be press-fit into the bone, a cavity is created in the bone to receive the geometry of the implant. For the press-fit method, it is important that this cavity closely fits the implant shape in order to hold the implant securely in the bone.
Currently, there are two common methods for creating this cavity in the bone for the implant. One method used is to remove bone material using a set of rasps or broaches and a mallet. The other method is to remove bone by milling the bone with a set of reamers, drills, or other revolving cutting tools. The method of this invention is that of milling the bone with cutting instruments. Examples of devices that use milling instruments to machine the bone include those found in U.S. Pat. Nos. 5,540,694 (De Carlo, Jr. et al.), U.S. Pat. No. 5,342,366 (Whiteside et al.), and U.S. Pat. No. 4,777,942 (Frey et al.).
Current milling instrumentation systems are unable to machine shapes that match the naturally occurring internal anatomy of the bone. Due to the nature of using revolving cutting tools, the implant designs driven by these instrumentation systems consist of rudimentary combinations of cones and cylinders. Preparing bone for implants of these basic shapes often causes one to compromise the fit of the implant to the bone. The implant cavity prepared with milling is very simple and the anatomy of the bone is much more complex. Implanting non-anatomic geometries requires removing more bone in some areas and less bone in other areas than is desirable. Using the broach/rasp system, more complex and anatomically correct implant shapes can be created in the bone, when compared to those geometries which can be milled into the bone using simple cone and cylinder instrumentation. Although broach/rasp systems can create a more anatomically correct shape in the bone, the act of repeatedly hammering the broach/rasp in and out of the bony canal is imprecise. On the other hand, previous milling systems were able to create accurate holes in the bone which were too simple to be anatomically correct. Anatomically correct implants typically include arcs or compound arcs having parallel or tapered sides. These arcs can be either symmetrically or asymmetrically placed relative to the bulk of the proximal implant geometry.